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Short Version

SQL Server is using 34 GB of RAM. But when query the Memory Consumption Report, the buffer pool size, and the ad-hoc query size, it only adds up to around 2 GB. What is the other 32 GB of RAM doing?

Pre-emptive: "You should cap the amount of RAM SQL Server can use." Lets say it is capped to x. That simply changes my question to, "What is the other x GB of RAM doing?"

Long Version

I have an instance of SQL Server that is consuming 32 GB of RAM:

enter image description here

That's not 32 GB of virtual memory; it's actually consuming 32 GB of physical memory (on the RAM chips) - known as "working set".

And it's not like it is shared with some other process. Essentially all of that is private to SQL Sever:

enter image description here

  • Private Working Set: 33,896,700 bytes

What is it doing with all that RAM?!

Bufferpool memory usage by database

So we query memory usage by database - as the buffer pool caches pages from the databases:

--Memory usage server wide
;WITH src AS
(
    SELECT
        database_id,
        COUNT_BIG(*) AS db_buffer_pages
    FROM sys.dm_os_buffer_descriptors
    --WHERE database_id BETWEEN 5 AND 32766
    GROUP BY database_id
)
SELECT
    CASE [database_id] WHEN 32767 THEN 'Resource DB' ELSE DB_NAME([database_id]) END AS [Database Name],
    db_buffer_pages AS BufferPages,
    db_buffer_pages /128.0 AS BufferMB
FROM src
ORDER BY db_buffer_pages DESC
OPTION(RECOMPILE, MAXDOP 1);

For a grand total of 4.5 MB of the 32 GB.

TempDB uses the most (1.4 MB), and the rest go down from there:

enter image description here

5 MB of 32 GB - doesn't account for much

Yes that might seem low - but that's likely because i called DBCC DROPCLEANBUFFERS first.

Query plan cache

Next we query the Query Plan Cache. All those T-SQL statements have to be compiled into a huge plan, and those plans are cached in RAM.

--Server-wide memory usage of plan cache
SELECT
    [cacheobjtype], ObjType,
    COUNT(1) AS Plans,
    SUM(UseCounts) AS UseCounts,
    SUM(CAST(size_in_bytes AS real)) / 1024.0 / 1024 AS [SizeMB]
FROM sys.dm_exec_cached_plans
--where [cacheobjtype] = 'Compiled Plan' and [objtype] in ('Adhoc', 'Prepared')
GROUP BY CacheObjType, ObjType
ORDER BY SizeMB DESC
OPTION(RECOMPILE, MAXDOP 1)

Now we can see how much memory is used to store various query plans:

cacheobjtype ObjType Plans UseCounts SizeMB
Compiled Plan Proc 3 4 0.21875
Parse Tree UsrTab 1 1 0.03125
Parse Tree View 1 6 0.0234375

For a grand total of 250 KB - far short of the missing 32 GB.

Note: Yes that might seem low - but that's likely because i called DBCC FREEPROCCACHE first.

Memory Consumption Report

The above queries show me RAM used by:

  • the buffer pool (to cache in memory database pages from disk)
  • the query plan cache

And that's really all there is. But SQL Server does provide a Memory Consumption Report:

This report provides detailed data on the memory consumption of components within the Instance

Narrator: "It doesn't"

The report is a little hard to read:

enter image description here

But in the end the breakdown is:

  • MEMORYCLERK_SOSNODE: 131,832 KB
  • MEMORYCLERK_SOSMEMMANAGER: 71,464 KB
  • USERSTORE_DBMETADATA: 67,432 KB
  • USERSTORE_SCHEMAMGR: 55,784 KB
  • MEMORYCLERK_SQLSTORENG: 54,280 KB
  • MEMORYCLERK_SQLBUFFERPOOL: 30,576 KB
  • Others: 145,056 KB

That gives a grand total of: 556,424 KB → 544 MB

Even if we round that up to 1 GB: it's still a far cry from 32 GB.

So where is the memory going?

Yes, i can cap SQL Server to 25 GB of RAM. But then that would just change my question to:

What is SQL Server is using 25 GB of RAM for; where is the memory going?

Because this sounds an awful lot like a memory leak to me.

  • Server: SQL Server 2012 (SP3) (KB3072779) - 11.0.6020.0 (X64)

Server up-time

Can query for server up-time (creation of tempdb):

--Use creation date of tempdb as server start time

SELECT SERVERPROPERTY('SERVERNAME') AS ServerName, create_date AS ServerStartedDate FROM sys.databases WHERE NAME='tempdb';

  • Server Started Date: 2021-12-21 15:46:26.730

CLR Assemblies

SELECT * FROM sys.assemblies
name principal_id assembly_id clr_name permission_set permission_set_desc is_visible create_date modify_date is_user_defined
Microsoft.SqlServer.Types 4 1 microsoft.sqlserver.types, version=11.0.0.0, culture=neutral, publickeytoken=89845dcd8080cc91, processorarchitecture=msil 3 UNSAFE_ACCESS 1 2012-02-10 20:15:58.843 2012-02-10 20:15:59.427 0

Linked servers

select provider, provider_string from sys.servers

provider provider_string
SQLNCLI NULL
MSIDXS NULL
search.collatordso NULL
DB2OLEDB Package Collection=▒▒▒▒▒▒▒▒;Network Address=▒▒▒▒▒;Network Port=50000;Connection Timeout=0;
9
  • How long has the instance been up? Is it using any linked servers? Also 2012 SP3 has been out-of-support for over 3 years. Jan 9 at 20:49
  • You're not even on the latest build of 2012, there were 10 CUs and then SP4 and finally another security update after the build you are on. Are you using SQLCLR? Jan 9 at 21:02
  • @DavidBrowne-Microsoft Server started 12/21/2021. There are some linked servers configured, yes.
    – Ian Boyd
    Jan 9 at 22:14
  • @Charlieface The only registered assembly (SELECT * FROM sys.assemblies) is Microsoft.SqlServer.Types.
    – Ian Boyd
    Jan 9 at 22:15
  • 1
    The middle two of those OleDb providers I don't usually see usually loaded into SQL Server's long-running process, and may be the cause of a memory leak. What kind of windows do you have to apply service packs, and/or restart the instance? Jan 9 at 23:01

1 Answer 1

5

If you're running DBCC DROPCLEANBUFFERS before checking the memory usage, it will only show a small amount of memory being used by SQL Server, but the freed memory won't be released back to the OS by running this command.

DBCC DROPCLEANBUFFERS will remove all clean buffers from the buffer pool, however, sqlservr.exe will retain the memory it has previously been allocated and reuse this memory to begin allocating pages in the buffer pool again immediately after DBCC DROPCLEANBUFFERS is executed. If you're encountering low memory conditions on the server, sqlservr.exe may begin releasing memory back to the OS, unless you have Lock Pages in Memory enabled.

These memory consumption queries/reports won't report anything of interest immediately after DBCC DROPCLEANBUFFERS because the allocated memory for the process is now essentially unused. To see how SQL Server is using its memory allocation, run these queries/reports prior to running DBCC DROPCLEANBUFFERS (which really shouldn't be run at all on a production server).

NOTE: You can force SQL Server to release memory back to the OS by dropping max server memory after DBCC DROPCLEANBUFFERS. This isn't typically instant but with mostly unused pages allocated to the sqlservr.exe process, it should release the memory fairly quickly.

2
  • This is an important point I missed above. Jan 9 at 23:40
  • The SysInternals tool VMMap can issue the command to empty the working set; which did cause the RAM to release back to the OS. I guess i will have to wait until it reaches 32 GB again, and re-post the numbers.
    – Ian Boyd
    Jan 10 at 2:23

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